Remove i128 and u128 from improper_ctypes_definitions

Rust's 128-bit integers have historically been incompatible with C [1].
However, there have been a number of changes in Rust and LLVM that
mean this is no longer the case:

* Incorrect alignment of `i128` on x86 [1]: adjusting Rust's alignment
  proposed at rust-lang/compiler-team#683,
  implemented at rust-lang#116672.
* LLVM version of the above: resolved in LLVM, including ABI fix.
  Present in LLVM18 (our minimum supported version).
* Incorrect alignment of `i128` on 64-bit PowerPC, SPARC, and MIPS [2]:
  Rust's data layouts adjusted at
  rust-lang#132422,
  rust-lang#132741,
  rust-lang#134115.
* LLVM version of the above: done in LLVM 20
  llvm/llvm-project#102783.
* Incorrect return convention of `i128` on Windows: adjusted to match
  GCC and Clang at rust-lang#134290.

At [3], the lang team considered it acceptable to remove `i128` from
`improper_ctypes_definitions` if the LLVM version is known to be
compatible. Time has elapsed since then and we have dropped support for
LLVM versions that do not have the x86 fixes, meaning a per-llvm-version
lint should no longer be necessary. The PowerPC, SPARC, and MIPS changes
only came in LLVM 20 but since Rust's datalayouts have also been updated
to match, we will be using the correct alignment regardless of LLVM
version.

`repr(i128)` was added to this lint in [4], but is also removed here.

Part of the decision is that `i128` should match `__int128` in C on
platforms that provide it, which documentation is updated to indicate.
We will not guarantee that `i128` matches `_BitInt(128)` since that can
be different from `__int128`. Some platforms (usually 32-bit) do not
provide `__int128`; if any ABIs are extended in the future to define it,
we will need to make sure that our ABI matches.

Closes: rust-lang#134288
Closes: rust-lang#128950

[1]: rust-lang#54341
[2]: rust-lang#128950
[3]: rust-lang/lang-team#255 (comment)
[4]: rust-lang#138282

Author: tgross35

core/src/primitive_docs.rs

@@ -1428,6 +1428,18 @@ mod prim_i64 {}
 #[rustc_doc_primitive = "i128"]
 //
 /// The 128-bit signed integer type.
+///
+/// # ABI compatibility
+///
+/// Rust's `i128` is expected to be ABI-compatible with C's `__int128` on platforms where the type
+/// is available, which includes most 64-bit architectures. If any platforms that do not specify
+/// `__int128` are updated to introduce it, the Rust `i128` ABI on relevant targets will be changed
+/// to match.
+///
+/// It is important to note that in C, `__int128` is _not_ the same as `_BitInt(128)`, and the two
+/// types are allowed to have different ABIs. In particular, on x86, `__int128` and `_BitInt(128)`
+/// do not use the same alignment. `i128` is intended to always match `__int128` and does not
+/// attempt to match `_BitInt(128)` on platforms without `__int128`.
 #[stable(feature = "i128", since = "1.26.0")]
 mod prim_i128 {}
 
@@ -1458,6 +1470,8 @@ mod prim_u64 {}
 #[rustc_doc_primitive = "u128"]
 //
 /// The 128-bit unsigned integer type.
+///
+/// Please see [the documentation for `i128`](prim@i128) for information on ABI compatibility.
 #[stable(feature = "i128", since = "1.26.0")]
 mod prim_u128 {}